1. Field of the Invention
The present invention relates to a rotary hydraulic pressure control valve, particularly, it relates to a hydraulic pressure control valve which is preferable for controlling a feed oil pressure to a steering assisting hydraulic cylinder at hydraulic power steering.
2. Description of the Prior Art
A hydraulic power steering apparatus is designed to assist a power required for operating a steering wheel for steering by means of a hydraulic power produced by a hydraulic cylinder provided in a steering mechanism. In the hydraulic power steering apparatus, an input shaft linked to the steering wheel and an output shaft linked to the steering mechanism are coupled coaxially via a torsion bar, and responsive to a steering torque applied to the steering wheel, the relative angular displacement according to torsion of the torsion bar is produced between the two shafts. At joint of the two shafts, a cylindrical casing which rotates in interlocking motion with one shaft, and a rotary hydraulic pressure control valve inserted thereinto and including a valve body which rotates in interlocking motion with the other shaft are formed to control, by the operation of the hydraulic pressure control valve, feed oil pressure to the hydraulic cylinder responsive to the direction and magnitude of the relative angular displacement, namely, the direction and magnitude of the steering torque applied to the steering wheel.
In the hydraulic pressure control valve, the valve body is constituted by forming a plurality of grooves extending axially and equally spaced around the periphery near the joint end of either input or output shaft. The casing provided with the same number of grooves equally spaced around its inner surface is secured to the other joint end, and the valve body is inserted into the casing. The valve body and casing are so arranged that, when the relative angular displacement is not produced between the input and output shafts, the both grooves are staggered circumferentially, and portions communicating with the adjacent grooves and having the same areas are formed on the lateral sides of respective grooves. The hydraulic pressure control valve is constructed in a well known manner such that, for example, while the grooves in the valve body are connected alternately to a hydraulic pump of a high pressure source and an oil tank of a lower pressure source, the grooves in the casing are connected alternately to two oil chambers of the hydraulic cylinder. That is, the two oil chambers of the hydraulic cylinder are brought in communication with the high pressure source and low pressure source via the communicating portions on both sides of the grooves of the casing to which the respective oil chambers are connected. When the steering torque is applied to the steering wheel and the relative angular displacement is produced between the input and output shafts, in the groove connected to one oil chamber, communicating areas with the low pressure source are reduced and those with the high pressure source are increased, on the contrary, in the groove connected to the other oil chamber, communicating areas with the high pressure source are reduced and those with the low pressure source are increased. As a result, between the two oil chambers, the pressure difference in which the former oil chamber is larger than the latter oil chamber is produced and the steering assisting power is obtained responsive thereto.
Now, magnitudes of the power required for steering a motor vehicle are responsive to those of road reaction force acting on wheels. At a standstill and in the driving condition such as low speed driving where the reaction force is large, a large steering power is required, while in the driving condition such as high speed driving where the reaction force is small, a relatively small steering power is required. Thus, in the power steering apparatus, as shown in FIG. 1, when the steering torque applied to the steering wheel is less than a predetermined value T.sub.1, the steering assisting power is hardly produced, giving the same rigidity as the manual steering to enhance the rectilinear stability at high speed driving. While, when the steering torque exceeds the other predetermined value T.sub.2 larger than the value T.sub.1, the steering assisting power increasing suddenly against the increment is produced to reduce the steering power required for operating the steering wheel as low as possible. Furthermore, with respect to steering torques between the values T.sub.1 and T.sub.2, the steering assisting power which increases gradually against the increment must be produced. That is, desirable increment characteristics in the power steering apparatus is a so-called two-step characteristics having two increment variations at the values T.sub.1 and T.sub.2.
As described hereinabove, the steering assisting power corresponds to the pressure difference produced between the two oil chambers of the hydraulic cylinder, and the pressure difference changes responsive to areas of the communicating portions in the hydraulic pressure control valve. Thus, the two-step characteristics aforementioned is realized when the areas of communicating portions change rapidly until reaching the predetermined value and change gradually in its neighborhood thereafter, in the case where the relative angular displacement produced between the valve body and casing responsive to the steering torque applied to the steering wheel is small, and when the relative angular displacement is larger, change rapidly against the increment.
Therefore, various hydraulic pressure control valves designed to realize the two-step characteristics in the power steering apparatus, by forming notches at the corners between side walls of the groove and the inner surface of the casing or the periphery of the valve body to obtain the variable states of communicating areas as aforementioned, have been proposed hitherto.
Among these, as the control valve capable of realizing the aforesaid variable states relatively faithfully, for example, notches shown in FIG. 2 are formed therein.
In the figure, the numeral 1 indicates a casing and 2 denotes a valve body. In the casing 1, rectangular sectional grooves 5 extending axially are formed around the inner surface, and in the valve body 2, the similar grooves 6 are formed around its peripheral surface. The grooves 5 and 6 are positioned so as to be communicated one another via communicating portions 8, 8 having the same area on respective lateral sides, when the relative angular displacement between the casing 1 and valve body 2 is not produced.
At the corners between the inner circumferential surface of the casing 1 and side walls of the groove 5, notches 50 which keep communicating areas in the communicating portions 8, 8 substantially unchanged within the predetermined range against the increment of the relative angular displacement produced between the casing 1 and valve body 2 are formed. As shown in the figure, the notch 50 includes a first portion 50a which is a circular arc or a straight line parallel to the inner surface of the casing 1 and intersecting the side wall of the groove 5, and a linear second portion 50b which intersects with the first portion 50a and the inner surface of the casing 1 approximately at right angles and linking the two. The notches 50 may be formed at the corners of the valve body 2 opposing the corners aforementioned across the communicating portions 8.
FIG. 3 is an explanatory view of the variable state of areas of the communicating portion 8 having the notch 50. The areas of communicating portion 8 reduce rapidly until the corner of the valve body 2 reaches the position indicated by the broken line in the figure, responsive to the relative angular displacement produced between the casing 1 and the valve body 2. Thereafter, until the corner reaches the position indicated by the one-dot chain line in the figure, the areas of communicating portion 8 depend upon the radial depth b in the first portion 50a, an do not substantially change against the increment of the relative angular displacement, since the portion 50a is in parallel to the peripheral surface of the casing 1. When the relative angular displacement increases further, the areas of communicating portion 8 tend to reduce rapidly against the increment of the relative angular displacement since it depends upon the circumferential width s between the corner and the second portion 50b. Accordingly, by combining the notches 50 and the well-known notches which are formed separately therefrom and in which the communicating areas reduce gradually as the relative angular displacement increases, the two-step characteristics of the steering assisting power aforementioned is realized and a comfortable steering feeling can be obtained.
In the hydraulic pressure control valve, however, when the relative angular displacement between the casing 1 and valve body 2 is large and a gap s between the corner of the valve body 2 and the second portion 50b is small, for example, pressure oil which flows through the communicating portion 8 from the groove 5 toward the groove 6, as indicated by the white arrow in FIG. 3, after flowing into the notch 50 circumferentially along the casing 1, changes its flowing direction and flows through the gap between the second portion 50b and the corner of the valve body 2 approximately radially to the casing 1 into the groove 6, consequently, the flowing direction being changed suddenly almost perpendicularly in the notch 50. Thus in a motor vehicle including the power steering apparatus constituted by using such a hydraulic pressure control valve, for example, when the large steering operation is executed at a standstill or low speed driving such as entering the garage or putting aside the road and so on, there is a possibility that discordant flowing noises caused by the change of flowing direction occur, annoying the comfortable feeling as well as causing the driver misjudgement that as if abnormalities were occurring by hearing the flowing noises.